In recent years, to improve performance and reduce a manufacturing cost of a semiconductor device such as a solar cell or an MOS (Metal Oxide Semiconductor) transistor, an increase in diameter of a wafer formed of, e.g., silicon that is used as a substrate has advanced. Therefore, a single-crystal ingot which is grown by, e.g., CZ method and has a diameter of 200 mm (8 inches) or a diameter of 300 mm (12 inches) is manufactured, and an increase in diameter and an increase in weight advance.
Such a single-crystal ingot is manufactured by, e.g., a single-crystal manufacturing apparatus depicted in FIG. 2. FIG. 2 is a schematic view showing a general single-crystal manufacturing apparatus used in CZ method.
This general single-crystal manufacturing apparatus 20 is an apparatus for growing a single crystal 31 from a raw material melt 30 based on CZ method, and it is configured to accommodate in a main chamber 21a crucible 23 that contains the raw material melt 30 obtained by melting a polycrystal raw material, a heater 25 provided around the crucible 23, and a heat insulating material 26 provided around the heater 25.
In particular, components like the crucible 23, the heater 25 and the heat insulating material 26 that are heated are called hot zone components.
A pull chamber 22 configured to accommodate and take out a pulled single crystal 31 is connected to an upper end of the main chamber 21. Further, a gate valve 28 that opens/closes an opening portion at the upper end of the main chamber 21 is provided between the upper end portion of the main chamber 21 and the pull chamber 22. Furthermore, a single-crystal pulling mechanism (not shown) configured to wind up a wire 34 having a seed holder 33 attached at an end thereof is provided above the pull chamber 22.
To use such a single-crystal manufacturing apparatus 20 to manufacture the single crystal 31, a seed crystal 32 is held at an end of the seed holder 33, and the seed crystal 32 is immersed in the raw material melt 30 and gently pulled upward while rotating to grow the rod-like single crystal 31.
At this time, an inert gas such as Ar is circulated in the chamber while performing vacuum evacuation to discharge an oxide vaporized from a melt surface.
When pulling the single crystal is finished, the heater is turned off, the gate valve is closed, and the single crystal accommodated in the pull chamber is taken out. Further, after the hot zone components are cooled down, a pressure in the chamber is restored to a normal pressure, and then the hot zone components in the main chamber are dismantled. When the dismantlement of the hot zone components is finished, cleaning, replacement and others are carried out, then the hot zone components are again assembled, assembling the chambers, putting the raw material, vacuuming and melting a polycrystal raw material are carried out, and a single crystal is again pulled up.
In such manufacture of the single crystal based on CZ method, to achieve an improvement in productivity and a reduction in cost, an increase in single-crystal growth rate is regarded as one large means to make many improvements in conventional examples. However, an operation cycle of manufacture of a single crystal based on CZ method includes pulling the single crystal and the above many steps other than the pulling, and further greatly reducing a pulling time is difficult under existing circumstances. Therefore, it is considered that reducing a process time for the steps other than the pulling of the single crystal is effective for improving an operation efficiency, i.e., for improving an operating rate of the single-crystal manufacturing apparatus to raise productivity.
Except for the single crystal pulling step, proportions of melting of the raw material polycrystal before pulling the single crystal and a cooling time of the hot zone components are high. The cooling time of the hot zone components is determined based on a condition that carbon members such as the heater are cooled down so that they are not degraded even though they are in contact with oxygen in air when restoring a pressure in the main chamber to a normal pressure. Even in case of manufacture of a single crystal having a diameter of 200 mm (8 inches) and a straight body length of 1 m which is the current mainstream, this cooling time reaches approximately 8 hours in natural cooling, and it takes a little less than a half of the process time of the steps other than the pulling.
The cooling time of the hot zone components is precisely an idle period of the single-crystal manufacturing apparatus. Therefore, this cooling time eventually becomes a cause of a considerable reduction in operating rate of the single-crystal manufacturing apparatus. A demand for an increase in diameter of the single crystal shows no sign of slowing down, and manufacture of a large single crystal having a diameter of 300 mm (12 inches) or above is often carried out. In this case, a heat capacity of each hot zone component is greatly increased beyond a current capacity, and the cooling time is thereby prolonged, and a reduction in operating rate of the apparatus due to prolongation of the cooling time becomes a more serious problem.
A pamphlet of International Publication No. WO 01/057293 discloses a configuration that a cooling cylinder and a cooling auxiliary member configured to cool a single crystal immediately after pulling are arranged to surround the pulled single crystal. However, since the cooling cylinder is apart from a raw material melt remaining in a crucible, this configuration hardly contributes to a reduction in cooling time of each hot zone component after end of pulling the single crystal.
Furthermore, in Japanese Patent Application Laid-open No. 9-235173 (KOKAI), as a method for cooling hot zone components after pulling a single crystal, a cooling time of the hot zone components is reduced by circulating an inert gas having an ordinary temperature or a lower temperature in a main chamber, thereby improving an operating rate of a single-crystal manufacturing apparatus.
However, a single crystal manufactured based on the technology of that time has a diameter of approximately 200 mm and a straight body length of approximately 70 cm, and a polycrystal raw material required for manufacture of the single crystal has a weight of approximately 200 kg, but the straight body length of the single crystal to be manufactured is increased at the present day for an improvement in pulling efficiency, and the weight of the required polycrystal raw material is also increased to approximately 300 kg.
As described above, the hot zone components including the crucible grow in size and heat capacities thereof also rise as an amount of the polycrystal raw material that is molten into a raw material melt increases.
Therefore, it cannot be said that adopting a method for circulating a gas having an ordinary temperature or a lower temperature in the chamber like Japanese Patent Application Laid-open No. 9-235173 (KOKAI) alone is sufficient to cool the hot zone components having increased heat capacities in a short time, and a further reduction in cooling time has been demanded.